Elevator systems include an elevator car that moves vertically to carry passengers, cargo or both to various levels within a building or structure. There are different arrangements for propelling the elevator car and supporting it within a hoistway. Most systems include a brake that is used to stop the elevator car and hold it in a desired position.
In traction-based elevator systems, for example, an elevator machine assembly includes a motor, a drive for controlling operation of the motor and a traction sheave that is driven by the motor to cause desired movement of the elevator car. A load bearing assembly (e.g., round ropes or flat belts) supports the weight of the elevator car and follows the fraction sheave such that movement of the traction sheave causes corresponding movement of the elevator car. The brake operates on a brake rotor disk, which is coupled with the traction sheave, to control the speed at which the sheave rotates or to prevent movement of the sheave (and the elevator car) to meet the needs of a particular situation.
There are a variety of elevator brake arrangements that have been used. Typical brakes include mechanical springs that force an axially moveable plate against the brake rotor having brake lining material. The resulting friction between the movable plate and the lining material stops and holds the elevator in place. Engagement of the moveable plate is known in the art as “dropping the brake” and is typically the default condition. Releasing the brake is known in the art as “lifting the brake.” Typical arrangements include a solenoid for causing movement against the bias of the mechanical springs to move the moveable plate out of engagement. The force generated by the solenoid overcomes the force of the springs and pulls the moveable plate away from the brake rotor.
Using a solenoid for moving the plate is inherently unstable in that acceleration changes with a change in the gap between the moveable plate and the associated components (e.g., as the moveable plate moves out of or into engagement). Magnetic fields increase as the ferromagnetic parts (e.g., the moveable plate and a housing that supports the electromagnet) come closer together, tending to create an acceleration of the movable plate. The plate typically moves through an air gap of approximately 0.3 mm between an engaged (i.e., dropped) and a disengaged (i.e., lifted) position. If the magnetic field decays too quickly when dropping the brake, for example, then the movable plate is accelerated by the springs into contact with the brake rotor. The uncontrollable acceleration of the plate though the air gap and resulting contact with the brake rotor or housing can result in objectionable noise that can be heard within the elevator car.
Attempts to reduce such noise include the use of O-rings to dampen movement, reduce the impact, and reduce noise. Disadvantageously, the O-rings are subject to creep, stress relaxation and aging. Over time these factors degrade the O-rings causing a noticeable increase in noise, along with a reduction in the force that engages the brake. The increase in noise and reduction in engagement force ultimately requires readjusting the brake torque and replacing the O-rings to maintain the desired operation and noise dampening characteristics. Additionally, typical arrangements require initially setting the torque higher than otherwise required to compensate for the eventual degradation of the O-rings. Such over-design of an elevator brake introduces additional costs. Other known devices include the use of an elastomeric bumper or pad. Such devices also suffer from the limited life span associated with the O-rings.